Use less intrusive style for displaying unit flags

Those usually are not main priority for translators, so the goal is to focus more on comments and file location hints. Legend seems like a nice fit with smaller text and gray color instead of black, but of course it slightly kills semantics.

Another issue i currently have with this is how ugly it looks. For instance, a .po line like this
```po
#, fuzzy, python-format
```
doesn't add much value by showing the (pretty confusing) `#,` which in this case looks like a list of three items.

 1. Do we need to display `fuzzy` as flag again even if we have the checkbox right on top of it?
 2. Can the `#, ` be dropped or does this break other formats (haven't checked the ttk code there)?
 3. The multi-line thing outlined below

Consider
```po
#, python-format
#, fuzzy
```
which shows as `Flags: #, python-format , #, fuzzy`